Four radioisotopic immunoassays of free thyroxine compared

Determination of (+)-3,3',5-triiodo-L-thyronine (L-T3) from serum using a sequential injection analysis/immunosensor system

A sequential injection analysis/immunosensor system is proposed for the analysis of T3 in serum with a rate of 75 samples/hr. The immunosensor design is based on the physical immobilization of anti-T3 in carbon paste. The working concentration range of the immunosensor in a sequential injection analysis system is between 3.4 and 340 ng/mL with a limit of detection of 2.19 ng/mL. The system is very reliable and very easy to design and operate.

Serum thyroxine binding capacity-dependent bias in five free thyroxine immunoassays: assessment with serum dilution experiments and impact on diagnostic performance

OBJECTIVES

To assess the presence and magnitude of a serum thyroxine binding capacity (sBC)-dependent bias in five free thyroxine (FT(4)) immunoassays, compared with equilibrium dialysis (ED). The exhibited bias is confronted with clinical results from previous studies to evaluate its impact on FT(4) determination in sera with various sBC.

DESIGN AND METHODS

The sera of three pregnant women and three non thyroidal ill (NTI) patients were serially diluted in an inert buffer to progressively decrease the sBC. FT(4) values were measured in diluted and undiluted samples with the six assays.

RESULTS

As a function of increasing dilution performed on pregnancy sera, except for ED and Vitros ECi FT(4,) the other four FT(4) assay results decreased to different degrees, in the following order: two-step GammaCoat RIA< Elecsys< ADVIA:Centaur< Amerlex-MAB RIA. In sera from NTI patients, the decrease was more marked and found at high dilution with the Vitros ECi assay. Data from previous studies showed that FT(4) measured with biased assays were decreased only in samples from very severely NTI patients with low sBC and that FT(4) results in pregnancy sera with high sBC were not significantly biased.

CONCLUSIONS

The dilution test is a sensitive alarm to assess sBC-dependent bias in FT(4) assays. For all FT4 assays and particularly when a bias is observed, documentation should be sought on the diagnostic performance of the assay and supported by a detailed clinical study including samples with low sBC. Physicians should still be educated about the limitations of FT(4) assays.

Cranial irradiation and central hypothyroidism

Cranial irradiation causes thyrotropin (TSH)-releasing hormone (TRH) secretory abnormalities. TRH deficiency leads to abnormal glycosylation of TSH alpha and beta subunits and loss of the normal circadian pattern of TSH secretion (low in the afternoon, a surge in the evening, higher at night). This disruption results in either mixed hypothyroidism (raised TSH with abnormal secretory kinetics) or central hypothyroidism (abnormal secretory kinetics without raised TSH). Although primary hypothyroidism is more common in the general population and cancer survivors, the cumulative incidence of central and mixed hypothyroidism is high during the ten years after cranial irradiation. Monitoring for decline in free thyroxine (FT(4)) and rise in serum TSH, and early recognition using TSH surge and TRH tests, are clinically valuable. Early thyroid hormone replacement therapy to achieve serum FT(4) in the upper half of the normal range is crucial for maintaining optimal health and growth in cancer survivors.

Effects of ramipril on the hormone concentrations in serum of hypertensive patients

The effects of the angiotensin-converting enzyme inhibitor ramipril on thirteen endocrinological tests were evaluated. These tests comprised serum follitropin, lutropin, prolactin, thyrotropin, free thyroxine, total thyroxine, free triiodothyronine, parathyrin, cortisol, testosterone, sex hormone binding globulin, androstenedione and dehydroepiandrosterone sulphate. Eleven hypertensive outpatients, 9 men and 2 women, treated at the department of internal medicine in Turku University Central Hospital, received 5 mg of ramipril once a day for the study period of four weeks. The above mentioned endocrinological tests were performed before and at the end of the ramipril treatment. Ramipril decreased the value of free thyroxine statistically significantly, p = 0.011, from the mean value of 17.1 pmol/l to the mean value of 16.0 pmol/l when measured with Amerlex-MAB* free thyroxine kit. The mean within-subject difference was -1.10 pmol/l with a 95% confidence interval of -1.87 - -0.33 pmol/l. With the AutoDELFIA free thyroxine kit and with the reference method dialysis+RIA no effect was detected. Other endocrinological tests examined were not affected by ramipril. Since the decreasing effect of ramipril on free thyroxine was detected only with Amerlex-MAB* but neither with AutoDELFIA nor with dialysis+RIA, the effect was concluded to be analytical. The underlying mechanism and the component ultimately interfering with the analysis is unknown.